This invention relates to plasma etching processes, and in particular to a method for stopping the process at the point where a layer is completely etched through.
A great deal of development in integrated circuit fabrication is presently directed to producing smaller circuits which require very fine line definition. For example, in the silicon gate technology employed for MOS circuits used in microprocessors, it is now desired to achieve gate widths as small as 1.0.mu.. Similar fine line definition is also desirable for etching other layers, such as silicon dioxide and silicon nitride typically used for surface passivation, and aluminum used for ohmic contacts in integrated circuits.
As a result of this need for fine line definition, the attention of the industry has been increasingly directed to plasma etching processes. Basically, such processes involve placing the semiconductor in a chamber, introducing a gas therein and applying an electric field to the gas utilizing a voltage which alternates at an RF frequency so as to create a plasma. In the case of etching processes, the plasma includes highly reactive gas species which react with the layer to be etched to produce a volatile gas which can be pumped out of the chamber. The layer can be selectively etched by using a suitable mask such as a photoresist.
Such processes are in general capable of etching patterns with sufficient resolution for present and future generations of circuits. However, care must be taken to insure that the reaction continues just long enough to reproducibly achieve the precise line width desired. If there is too little etching, the pattern will not be completely removed in the selected areas, and if there is too much etching, undercutting will result (see, for example, Somekh "Introduction to Ion and Plasma Etching", Journal of Vacuum Science Technology, Vol. 13, No. 5 pp. 1003-1007 (September/October 1976)).
One standard approach is to determine the proper end point empirically based on the type of layer, thickness, number of wafers, power, gas flow and other factors. This approach, of course, requires a new determination for each system and etching condition. Further, variations in parameters can occur during the reaction which will affect the time of etching completion. A further approach is to detect emission spectra changes in gas reactants or products and stop the reaction when the appropriate spectral line indicated on a strip line recorder reaches a certain value. Although adequate, this approach also has the disadvantage of requiring a new detection scheme for each reaction. Further, the plasma chamber would often cloud up, interfering with the optical detection. Also, many reaction chambers are made of metal and cannot employ such detection means.
It is therefore a primary object of the invention to provide a plasma etching process including means for accurately detecting the completion of the etching reaction which is generally applicable regardless of the particular reaction and parameters.